1. Field of the Invention
The present invention relates to the field of Optical Transport Networks and in particular a method for interlayer protecting such networks. The present invention also relates to a node and a network implementing such a method.
2. Description of the Prior Art
ITU-T G.872 Recommendation (version 11/2001, which is incorporated herewith as a reference) defines the architecture of OTN (Optical Transport Network) also termed OTH (Optical Transport Hierarchy). The two terms are considered as equivalent for the purposes of the present invention. The OTN is made of three main layers: OTS (Optical Transmission Section), OMS (Optical Multiplex Section) and OCH (Optical Channel). Furthermore, ITU-T G.709 Recommendation (03/2003, which is also incorporated herewith as a reference) defines the OCH layer structure and the frame format of the optical channel at the ONNI (Optical Network Node Interface).
In this scenario, the present invention, focuses on the OTU/ODU layer. ITU-T G.709 foresees some information to manage these layers. For each layer, the transported information is made of payload and Over-Heads. Particularly the OCH layer, made of two main units (ODU and OTU), transports payload and associated Overhead information. The purpose of the OCH is to carry information to manage and to monitor the end-to-end connection through the network.
The Applicant has identified two pending technical problems. The first consists in allowing interlayer protocol communication between two different protection segments (i.e. OTU trail and ODU-SNC or OTU trail and ODU-Trail) or between two different levels of switches within the same protection layer and driven by the same protection mechanism: i.e. ODU-SPRing span switch and ODU-SPRing ring switch. The second consists in providing the ability to forward an OTU-Trail degradation indication (signal degrade condition) to the remote ODU-Trail termination function in charge to activate the switching action either in the ODU-Trail or in the ODU-SNC protection configurations.
A problem similar to the above first one has been identified in synchronous transport networks, typically SDH or SONET networks. For instance, a SDH network could be protected at a Server layer and at a Client layer. At a Client layer, a path is protected by providing an alternative route for connecting two end nodes. At a Server layer, each single section could be protected by a MS-Protections mechanism. If a fiber cut or a carrier failure occurres in a span, both the two mechanisms start to operate. No coordination action is performed between the protection mechanisms at Client and Server layers. Generally, the section protection mechanism (at Server layer) is the one starting its operation first. In the SDH technology, the above problem has been solved by the so-called Hold Off Time, briefly, HOT. According to it, a timer is set at a certain time frame (for instance at 100 millisec); upon an end node has received a signal fail (or degrade) indication, the timer is started; if the protection mechanism at Server layer succedes in solving the fail problem before the expiring of the timer time frame, no client layer mechanism is activated; in the negative, the client layer protection mechanism is activated.
The HOT prevents wrong interactions between different protection layers or protection segments but it introduces inefficiences in the traffic restoration due to the long switching-time achieved by using HOT solution. Furthermore, the procedure according to HOT is time consuming. In fact, when the failure cannot be overcome by the protection mechanism operating at Server layer, a rather long time (for instance at 100 millisec) is lost.
In view of the above, the Applicant has deemed that implementing in an optical network a mechanism which is similar to the HOT was not convenient and has devised a dynamic interlayer protection method overcoming the above problems.